The basic process steps of electrostatographic printing, such as xerography or ionography, are well known. Typically an electrostatic latent image is created on a charge receptor, which in a typical analog copier or “laser printer” is known as a photoreceptor. The suitably charged areas on the surface of the photoreceptor are developed with fine toner particles, creating an image with the toner particles which is transferred to a print sheet, which is typically a sheet of paper but which could conceivably be any kind of substrate, including an intermediate transfer belt. This transfer is typically carried out by the creation of a “transfer zone” of AC and DC biases where the print sheet is in contact with, or otherwise proximate to, the photoreceptor. Devices to create this transfer zone, such as corotrons, are well known.
Another condition which is known to be useful in a transfer zone is mechanical pressure between the print sheet and the photoreceptor: a certain amount of pressure can enhance transfer efficiency, image quality and “latitude” (the range of types of paper or other substrate which can be effectively printed on). To obtain such pressure, it is known to use a “bias transfer roll,” which is an electrically-biased roll urged against either a rigid photoreceptor drum or a back up roll inside a photoreceptor belt. The combination of mechanical pressure and electrical bias creates a suitable transfer zone in the nip between the bias transfer roll and the photoreceptor.
The present disclosure relates to a novel apparatus for creating suitable conditions in a transfer zone.